In a blood treatment apparatus with an extracorporeal blood circuit, such as a dialysis apparatus, numerous monitoring operations have to be performed to protect the patient or the operator from possible hazards. To this end, the blood treatment apparatus is equipped with a plurality of sensors that monitor various parameters such as pressures, temperatures, conductivity or flow and perform a control or a switching operation in dependence on the result. It is common that certain single parameters or limit values are entered into the apparatus by the operating personnel and that the apparatus monitors the adherence to the parameters and limit values during operation. Upon occurrence of an unacceptable deviation, the apparatus is transferred into a safety state and an issuing of an alarm calls the personnel to the apparatus to take the necessary measures for the treatment to be continued. Usually, a nurse takes care of a number of patients so that he or she cannot immediately give his or her attention to the patient or the apparatus every time an alarm occurs. It is common to differentiate the sounds of acoustic alarms so that in the event of a plurality of simultaneous alarms from different apparatus, a nurse can immediately determine which alarm or which apparatus has the higher priority.
In a blood treatment apparatus it is known to provide a controller for controlling and monitoring which receives and processes the signals from the sensors and is connected with a first alarm system. The high safety requirements for such an apparatus make it imperative to monitor the controller for the fulfillment of its function. This is effected by a supervisor, which also receives sensor signals and, using function redundancy, checks whether the controller responds appropriately in each individual case. The controller and the supervisor are processors that function independent from each other and mutually control each other. The supervisor is also equipped with an alarm system.
Generally, dialysis treatments are performed in dialysis centers. Typically, they have 20 to 50 treatment places distributed over a plurality of rooms. The attendant personnel performs the treatment according to a doctor's treatment schedule or on direct instructions. The part of the treatment schedule that is related to the apparatus is entered into the apparatus as setting parameters. The inputted parameters are monitored by the apparatus during the treatment. Upon an unacceptable deviation, a safe mode for the patient is assumed and the attendant personnel is called to the apparatus to take measures that allow the treatment to be continued. Usually, one nurse attends to several patients and cannot immediately give her or his attention to the patient or the apparatus on every alarm. To prevent this from resulting in hazards to the patient, the sound or the optical appearance of the alarm signals are varied according to the urgency of the treatment abnormality.
In a dialysis apparatus, concentrates and water are mixed to supply the dialyser with hemodialysis solution. Separated by a membrane, the blood flows in the dialyser in a direction opposite to the hemodialysis solution. In the process, contaminants are removed from the blood through the hemodialysis solution. Usually, one dialysis treatment session takes four to six hours. During this period of time, conditions may arise that cannot be controlled by the dialysis apparatus. Such conditions include, for example:                arterial pressure is too negative due to an obstruction in the arterial line of the extracorporeal circuit upstream of the blood pump;        venous pressure is too low due to a venous needle having slipped out of the patient access;        no water in the supply line;        concentrate container is empty;        and        no voltage supply.        
To detect such conditions that are hazardous to patients, dialysis apparatuses are equipped with two independent alarm systems so that they can still operate safely should one component be defective. The following functions are given in each alarm system:                storing the limit values for the parameters to be controlled;        transducers for the parameters to be monitored;        processing of measured values for the parameters to be monitored;        comparison of the measured values to the associated limit values;        control of the actors to assume a safe treatment mode (blood pump stop;        bypass of        the hemodialysis solution at the dialyser);        optical indication of the alarm condition; and        acoustic indication of the alarm condition.        
Usually, the acoustic alarm alerts the attendant personnel to the irregular state of the apparatus. It is irritating when an alarm condition has already been signaled by the first alarm system and the second alarm system also signals the same condition, since this suggests greater urgency to the attendant personnel although this is not necessary.
DE 199 01 288 A1 describes a device for monitoring loudspeakers with which the functionality of individual loudspeakers can be monitored by recording an acoustic signal emitted by the loudspeaker. Thus, the failure of a loudspeaker can be detected.